1. Field of the Invention
The present invention relates to a position detecting device and an image forming apparatus.
2. Description of the Related Art
In image forming apparatuses, in particular, in a tandem color machine, image forming units that form images of yellow (Y), cyan (C), magenta (M), and black (K), respectively, are disposed side by side. The images of the respective colors are superimposed one on top of another on an intermediate transfer belt to form a full color image. Thus, color misregistration may occur and cause deterioration in image quality.
As an approach to this problem, technologies has been proposed in which a mark on an intermediate transfer belt is read to detect speed of the intermediate transfer belt. For example, Japanese Patent No. 3344614 discloses a technology for, in reading a reference mark formed on a transfer belt using two sensors, offsetting an error inherent in the reference mark and realizing accurate speed detection by acquiring an average speed of the belt in a time equivalent to several times of rotation of a driving roll. Japanese Patent Application Laid-Open No. 2006-160512 discloses a technology for providing a highly accurate belt transfer device by, in detecting a mark with two sensors, paying attention to fluctuation in an error of a mark interval, calculating a mark-pitch change from phase difference fluctuation of signals from the two sensors, and reflecting the mark-pitch change on a speed calculation to accurately detect a surface linear speed of a belt even if an error occurs in a mark pitch on the belt and perform feedback control. In such technologies, in general, the sensors (detecting units) are fixed to a holding member to locate detection positions of the sensors on perpendiculars to a belt conveying direction including positions for fixing the sensors to the holding member.
In an image forming apparatus, fixing operation or the like inevitably involves a temperature rise. With the former technology, speed of an intermediate transfer belt can be detected; however, the intermediate transfer belt is expanded and contracted due to a temperature change due to fixing operation, which results in misregistration. That is, the mark set as the reference is read using the two sensors. However, the sensors for detecting the mark are located on perpendicular lines to the conveying direction of the belt including the positions for fixing the sensors to the holding member. Thus, when temperature changes (rises), the holding member that fixes and holds the sensors is expanded and a space between the two sensors changes. As a result, the positions of the sensors for detecting the mark also change, and it is impossible to accurately detect the mark on the intermediate transfer belt and accurately detect speed of the intermediate transfer belt. In the latter technology, when the temperature changes, a sensor interval changes because of expansion of parts that fix the sensors. Thus, it is impossible to accurately detect the mark and a control error occurs.